Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and enhance fuel economy. Specifically, the amount of EGR that is recirculated affects the NOx emissions and fuel economy. Increased exhaust gas recirculation may result in partial burning and misfires, thus, cause increased emissions, reduced driveability of the vehicle, and increased fuel consumption. Various sensors may be coupled in the engine system to estimate the amount of EGR being delivered to the engine. These may include, for example, various temperature, pressure, oxygen, and humidity sensors coupled to the engine intake manifold and/or the exhaust manifold.
One example approach for measuring EGR is shown by Kotwicki et. al. in U.S. Pat. No. 6,321,732. Therein, the EGR system includes pressure sensors mounted over a fixed orifice, where the pressure sensors are used to measure a change in pressure (e.g., delta pressure) across the orifice. The pressure sensors, referred to as delta pressure sensors, are used to measure the pressure difference across the orifice, which in turn is used to measure EGR and therefore control exhaust gas flow in the engine system. However, the delta pressure sensors are noisy, which in turn results in inaccurate EGR measurements that may result in the aforementioned issues. In addition, these sensors are installed in engine systems for the sole purpose of measuring EGR, thereby including such sensors n the engine systems may increase manufacturing cost.
Another example approach for measuring EGR is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. Therein, the EGR system includes an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent.
However, the inventors have identified potential issues with such an approach. One or more other engine operating parameters are also affected by the misrepresentation of EGR by the intake oxygen sensor in the presence of rich or lean (relative to stoichiometry) EGR. For example, in the presence of lean EGR, although the sensor measures a lower (absolute) amount of EGR, the sensor output correctly reflects the burnt gas fraction. As a result, any adjustments to spark timing, throttle position, and/or fuel injection that are based on the adjusted calibration coefficient may be incorrect. As another example, in the presence of rich EGR, the sensor does not provide an accurate estimate of how much excess fuel is in the EGR. As such, if the excess fuel is not properly accounted for in cylinder fuel injection, the fuel injected will be higher than desired. This may cause open-loop fueling of the engine to be richer than desired. In the closed-loop fuel control, the adaptive fuel may adapt for the excess fuel in the EGR but the adaptive correction will be attributed to a fuel system error. This may falsely trigger a fuel system error if the correction is above a threshold. The problem may be exacerbated due to a delay between the timing of fuel injection and the sensing of the fuel at the intake oxygen sensor. As a result, engine fueling and EGR control may be disrupted.
In one example, some of the above issues may be addressed by a method comprising during operation of an exhaust oxygen sensor in a variable voltage (VVs) mode where a reference voltage of the exhaust oxygen sensor is adjusted from a lower, first voltage to a higher, second voltage, adjusting engine operation based on an exhaust gas recirculation (EGR) amount estimated based on an output of the exhaust oxygen sensor and a learned correction factor based on the second voltage. In this way, the exhaust oxygen sensor may be used for EGR estimation and engine fueling accordingly compensated.
As an example, the exhaust oxygen sensor may be operated in a reference mode wherein the sensor is operated at the lower voltage, and an output of the exhaust oxygen sensor may be used for controlling air-fuel-ratio (AFR). However, under select conditions, the exhaust oxygen sensor may be transitioned from the reference mode to the variable voltage (VVs) mode, where the sensor is operated at the higher voltage and/or modulated between the lower voltage and higher voltage. In some examples, the higher voltage is a voltage at which water molecules are partially or fully dissociated at the exhaust oxygen sensor while the lower voltage is a voltage at which water molecules are not dissociated at the sensor. As such, the select conditions may include an engine non-fueling condition such as a deceleration fuel shut-off (DFSO) and an engine steady-state condition such as engine idle. During such conditions, the exhaust oxygen sensor may generate an output, which may be used to estimate an exhaust water concentration from fuel ethanol content and ambient humidity. Specifically, the ambient humidity may be estimated by operating the exhaust oxygen sensor in VVs mode during DFSO, and the fuel ethanol content may be estimated during engine idle condition when there is no EGR. As such, the ambient humidity and the fuel ethanol content may be referred to as a correction factor, and may further be used to estimate the amount of water in the exhaust when EGR is inactive.
Subsequently, during engine idle conditions, EGR may be recirculated from the exhaust passage to the intake passage, and the exhaust sensor may be operated in the VVS mode to estimate the total water concentration in the exhaust. As such, the total water concentration may include an extra amount of water that directly correlates to the amount of EGR that is recirculated, for a given fuel composition, for example. Thus, by subtracting the correction factor from the total water concentration, the amount of EGR that is recirculated may be estimated.
In this way, the exhaust oxygen sensor may be used to correct for variations arising due to changing fuel composition and ambient humidity and further used to estimate the amount of EGR being recirculated in the system. By correcting the sensor output appropriately to compensate for the effects fuel composition and ambient humidity, a more accurate EGR estimation can be provided by the sensor, thereby improving engine fueling and EGR control. By extending the functionality of the exhaust oxygen sensor (which may be used for AFR estimation in the reference mode) in the VVs mode, the same sensor may be used to estimate all of fuel ethanol content, ambient humidity, and water concentration in the exhaust, thus eliminating the need for additional sensors for measuring each of these factors, and thus reducing manufacturing costs. It may be appreciated that the sensor may not be continuously operated in the VVs mode, but returned to the reference mode after estimating the correction and water levels during the select conditions. Thus, the integrity of the exhaust oxygen sensor may be maintained by reducing sensor degradation, for example.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.